This invention relates to an image display device and an image display apparatus employing a reflection type spatial light modulator and, more particularly, to such image display device and image display apparatus in which the apparatus may be reduced in weight and lowered in production costs while the image displayed may be improved in contrast.
So far, a variety of image display devices and image display apparatus produced using these display devices have been proposed.
[1] Spatial Light Modulator
The spatial light modulator (SLM) is such a device in which, as image signals are incident thereon, the incident light is modulated, from one pixel to another, based on image data corresponding to the image signals. The spatial light modulator (SLM) may be classified into a transmission type in which the light transmitted through the spatial light modulator is modulated, and a reflection type in which the light reflected by the spatial light modulator is modulated.
The reflection type spatial light modulator is constructed by e.g., a liquid crystal or a digital micro-mirror. In particular, the device formed using the liquid crystal is termed a liquid crystal type spatial light modulator.
The liquid crystal may be classified into an optical rotatory (polarization light guide) mode type, a birefringence mode type, a light scattering mode type and a light absorption mode type. The liquid crystal used in general may be enumerated by a TN liquid crystal, employing the optical rotatory (polarization light guide) mode type twisted nematic (TN) operational mode, an STN liquid crystal employing a birefringence operational mode type super-twisted nematic (STN) operational mode and an FLC type liquid crystal employing the ferroelectric liquid crystal (FLC) mode.
These reflection type spatial light modulator, modulating the state of polarization, may be enumerated by a liquid crystal spatial light modulator of perpendicular orientation, employing the TN crystal, an anti-ferroelectric liquid crystal spatial light modulator and a birefringence mode spatial light modulator employing the TN liquid crystal, in addition to the ferroelectric liquid crystal spatial light modulator.
[2] Reflection Type FLC Spatial Light Modulator
The structure and the operating principle of the reflection type FLC spatial light modulator, in the reflection type spatial light modulator modulating the state of polarization, is explained.
Referring to FIGS. 1A to 1C, the reflection type FLC spatial light modulator is made up of a pair of electrodes and a liquid crystal material 105 inserted therebetween. An electrode part shown on an upper part of FIG. 1 includes a glass substrate 101A, a transparent electrode material 102A on an inner side (a lower side) thereof, and an film of orientation (a film the liquid crystal molecules of which have been aligned in one direction such as by rubbing) 103A on a further inner side (a further lower side) thereof. The other electrode part, shown on the lower side, is made up of a silicon substrate 101B, an aluminum electrode 102B, shown on an inner side (an upper side) thereof, and a film of orientation 103B, shown on a further inner side (further upper side) thereof. The aluminum electrode 102B also operates as a reflection film. On an outer (upper) side of the glass substrate 101A of the upper electrode part is arranged a polarizer 104.
FIG. 1A shows the state of the first voltage direction in which the voltage of a first direction is applied to the transparent electrode material 102A and to the aluminum electrode 102B. FIG. 1B shows the state of the second voltage direction in which the voltage of a second direction opposite to the first direction is applied to the transparent electrode material 102A and to the aluminum electrode 102B.
Referring to FIG. 1C, the liquid crystal material 105 exhibits no birefringence effect with respect to the incident polarized light in the state of the first voltage direction, however, in the state of the second voltage direction, it exhibits a birefringence effect with respect to the incident polarized light.
Since a beam of polarized light 107A, incident via polarizer 104, is transmitted through the liquid crystal material 105, under the condition of the first voltage direction, shown in FIG. 1A, and reaches the aluminum electrode 102B without changing the state of the wave polarization, since the liquid crystal material 105 exhibits no birefringence effect under this condition. A polarized light beam 107B, reflected by the aluminum electrode (reflecting film) 102B, is again transmitted through the liquid crystal material 105 to reach the polarizer 104 without changing the state of the wave polarization. That is, the light of the same state of wave polarization as that of the incident light is returned to the polarizer 104. Consequently, the light reflected by the aluminum electrode (reflecting film) 102B is obtained via polarizer 104 as the outgoing light.
On the other hand, under the condition of the second voltage direction, shown in FIG. 1B, the polarized light beam 107A, incident via polarizer 104, is transmitted through the liquid crystal material 105 and thereby subjected to birefringence effects so as to be changed from the state of linear polarization to that of circular polarization to generate a circular polarized light beam 107B. This circular polarized light beam 107B is reflected by the aluminum electrode (reflecting film) 102B and has its direction of rotation of the polarized light reversed by this reflection. The circular polarized light beam 107B, having the direction of rotation reversed, is re-transmitted through the liquid crystal material 105 so as to be subjected to the birefringence effect and so as to be thereby turned into a linear polarized light beam. This linear polarized light beam is perpendicular to the direction of polarization of the polarizer 104 and hence is not transmitted through the polarizer 104.
That is, in this reflection type FLC spatial light modulator, xe2x80x98white displayxe2x80x99 and xe2x80x98black displayxe2x80x99 dominate in a portion of the state of the first voltage direction and in a portion of the state of the second voltage direction, respectively.
[3] Projection Type Image Display Device Employing Reflection Type Spatial Light Modulator
In a routine reflection type spatial light modulator, for example, a projection type image display apparatus including e.g., a reflection type TN liquid crystal panel, the illuminating light projected from a lamp light source 201 falls on an illuminating optical system 202 having the functions of correcting the cross-sectional profile of a light beam, uniforming the intensity and controlling the angle of divergence, as shown in FIG. 2. This illuminating optical system 202 may be provided with a P-S polarization converter, not shown. This P-S polarization converter is an optical block for aligning the illuminating light in the non-polarized state into the P-polarized light or into the S-polarized light at an efficiency of 50% or higher.
In an embodiment, shown here, the illuminating light, transmitted through the illuminating optical system 202, is in a state of polarization in which the electrical vector is oscillated along a direction perpendicular to the drawing sheet, that is, in a state of S-polarized light with respect to the reflecting surface of a dichroic mirror reflecting the red light. That is, the illuminating light, emitted by the illuminating optical system 202, has only its red light component deflected 90xc2x0 in its proceeding direction, by the dichroic mirror 203 reflecting the red light. This red light component then is reflected by a mirror 204 to fall on a polarizing beam splitter (PBS) for red light 210.
The red light beam, incident on the PBS 210, has only its S-polarized component reflected by a dielectric film 210a of the PBS 210 to fall as incident polarized light on a reflection type TN liquid crystal panel 213 for red light. The illuminating light, reflected by this reflection type TN liquid crystal panel 213 for red light 213 as it is modulated in its state of polarization, again falls on the dielectric film 210a of the PBS 210 where it is detected so that only the P-polarized light is transmitted therethrough. Thus, polarization modulation is changed to luminance modulation. The illuminating light, now changed to the luminance modulation, falls on a cross-dichroic mirror 209.
The illuminating light, transmitted through the dichroic mirror 203, reflecting the red light, falls on a next arranged dichroic mirror 205 reflecting the green light. This dichroic mirror 205 reflects only the green light, while transmitting the remaining blue light component therethrough. The so separated green and blue light components have only the respective S-polarized light components reflected by the PBS 211 and the PBS 212, as with the red light, described above, so as to fall on a reflection type TN liquid crystal panel for green light 214 and on a reflection type TN liquid crystal panel for blue light 215, respectively.
The illuminating light, reflected by the reflection type TN liquid crystal panel for green light 214 and by the reflection type TN liquid crystal panel for blue light 215, as it is modulated in the state of modulation, again falls on dielectric films 211a, 212a of the PBS 211 and the PBS 212, respectively, where the light is detected to transmit only the P-polarized light to change the modulation from polarization modulation to luminance modulation. The outgoing light beam, now changed to luminance modulation, falls on a cross-dielectric prism 209.
In this image display device, the red, green and blue light components, modulated by the reflection type TN liquid crystal panels 213 to 215 for respective colors, depending on the displayed image, are synthesized in the cross-dielectric prism 209 to fall on a projection optical system 208 to form an image on a screen 216.
[4] Illuminating Device for Reflection Type Spatial Light Modulator
As the illuminating device for the reflection type spatial light modulator, there is an illuminating device described in Japanese Laying-Open Patent Publication H-9-189809, and shown herein in FIGS. 3 and 4. In this illuminating device, the readout light, radiated from an illuminating light source, not shown, falls on hologram color filters 303r, 303g and 303b, via a coupling prism 305 and a glass substrate 304, as shown in FIG. 3.
It should be noted that the hologram color filters 303r, 303g and 303b are volume hologram lenses for red light, green light and for blue light, respectively, and are each formed by layered micro-sized lenses for respective colors, each being of approximately a size corresponding to a pixel size and each having an interference fringe burned on laser light exposure at the outset. The xe2x80x98size corresponding to a pixel sizexe2x80x99 means that three pixels, namely a pixel of R (red), a pixel of G (green) and a pixel of B (blue), are assorted into a set. The hologram color filters 303r, 303g and 303b converge the red, green and blue light components in the spectrum of the readout light on associated color pixel electrodes 313r, 313g and 313b on a pixel electrode layer 313, respectively, through a cover glass 302 of a reflection type liquid crystal panel, a common electrode 318, a film of orientation 317, a liquid crystal layer 316, a film of orientation 315 and a dielectric mirror film 314.
These hologram color filters 303r, 303g and 303b exhibit dependency on polarization characteristics of the incident light. That is, of the incident light on the hologram color filters 303r, 303g and 303b, mainly the S-polarized light is diffracted, such that the P-polarized light is lower in diffraction efficiency than the S-polarized light. The reason is that, by strict solution of the xe2x80x98coupled wave theoryxe2x80x99, there is produced a difference between the diffraction efficiency of TE (S-polarization) and that of TM (P-polarization) if, in the case of the thicker reflection type hologram, a value t/{circumflex over ( )} is 1 to 5, where t is a hologram thickness and {circumflex over ( )} is a pitch of the interference fringe in the hologram, as shown in FIG. 4, such that the S-polarization is larger than the P-polarization by approximately 45% at the maximum than the S-polarization. For reference, see M. G. Moharam and T. L. K. Gayload: Rigorous Couple-Wave Analysis of Planar Grating Diffraction, J. Opt. Soc. Am. 71, 811-818 (1977), M. G. Moharam and T. K. Gayload: Rigorous Couple-Wave Analysis of Grating Diffraction E-Mode Polarization and Losses, J. Opt. Soc. Am. 73, 451-455 (1983).
In the readout light, obliquely incident on the hologram color filters 303r, 303gand 303b, mainly the light of the S-polarized component, is diffracted and falls perpendicularly on the liquid crystal layer 316. In this illuminating light, the light reflected as its direction of polarization is modulated 90xc2x0 (P-polarized component) exits the hologram color filters 303r, 303g and 303b perpendicularly without undergoing scarcely any diffraction because of the low diffraction effects.
The illuminating light, reflected by the dielectric mirror film 314, falls on a projection lens, not shown, whereby the light forms an image on a screen, not shown.
[5] Polarization Selective Holographic Optical Device
There are a number of techniques for realization of the polarization selective hologram optical device. As disclosed for example in U.S. Pat. No. 5,161,039, there is a hologram optical device comprised of a mixed material of a photo-curable resin or a thermosetting resin with a liquid crystal material, sandwiched and sealed between a pair of glass plates.
This hologram optical device is prepared by the following process. First, the laser light is subjected to interference on a panel having the aforementioned mixed material sealed therein. The interference fringes so produced are formed by many photons being present in a light portion and by only a lesser amount of photons being present in a dark portion. In the portion with a high photon energy, that is in the light portion of the interference fringes, the resin is cured and coalesces under the light or thermal energy, as a result of which two areas, namely a resin layer and a liquid crystal layer, are formed, with the liquid crystal material being left in the dark portions of the interference fringes.
The operating principle of the polarization selective holographic optical device, thus formed, is now explained. Of the two areas, formed as described above, the resin layer is optically isotropic, whereas the liquid crystal area is anisotropic, that is it exhibits birefringence. On the other hand, the refractive index of the resin layer n1 is approximately equal to the refractive index n0 of the liquid crystal layer. Thus, in the light incident on this holographic optical device, the light the direction of polarization of which corresponds to the ordinary light ray of the liquid crystal layer, experiences but little difference in the refractive index between the resin layer and the liquid crystal layer, so that the phenomenon of diffraction is manifested only on extremely rare occasions. Conversely, the polarized light component, the direction of polarization of which is perpendicular to the ordinary light ray of the liquid crystal layer, is subjected to periodic refractive index modulation, due to difference between the refractive index n1 of the resin layer 1 and the refractive index of the extraordinary light ray ne, thus producing diffraction effects.
Recently, researches in holographically-formed polymer dispersed liquid crystals H-PDLC, obtained on mixing monomers undergoing optical polymerization and liquid crystal molecules together in order to form interference fringes by a holographic technique, are proceeding briskly.
This is a technique derived from the optically induced phase separation xe2x80x98PDLCxe2x80x99 as discovered towards the middle of eighties (for reference, see Crawford G. P. and Zumer S., in Liquid Crystals in Complex Geometries, Ulor and Francis, London (1996)). The technique of preparation and the operating principle of this H-PDLC are hereinafter explained.
First, a mixture of e.g., liquid crystal molecules, a monomer (pre-polymer), sensitizing dyestuffs and a reaction initiator are sandwiched between a pair of glass plates and sealed in situ. The resulting assembly is exposed to interference fringes formed by the laser light. Then, in the light area of the interference fringes, photopolymerization is initiated to turn the monomer into a polymer. The result is that distribution in the monomer concentration is produced in the light and dark portions of the interference fringes to cause monomer migration from the dark area to the light area. Thus, there is produced, on phase separation, a periodic structure, comprised of a light area rich in polymer concentration and a dark area rich in liquid crystal molecules. As the next stage, the liquid crystal molecules are arrayed at right angles to the polymer phase. Although the mechanism of this phenomenon is not known precisely, a variety of pertinent researches are now going on (see, for example, C. C. Bowley, A. K. Fontecchio, and G. P. Crawford. Proc. SID XXX, 958 (1999)).
Subsequently UV light is illuminated to carry out a fixing process. With the holographic optical device, thus produced, as with the above-mentioned holographic optical device disclosed in U.S. Pat. No. 5,161,039, the refractive index of the polymer layer is approximately equal to the refractive index for the ordinary light ray of the polymer layer, while the refractive index of the polymer layer differs from the refractive index for the extraordinary light ray of the polymer layer, so that the optical device operates as a polarization selective holographic optical device.
[6] Techniques of Application of Holographic Optical Device
The conventional instances of application of the holographic optical device is now explained. Among the instances of application, there are an optical switch, a reflecting plate for an image display device and a polarization converter for a projection type image display device. These are now explained specifically.
[6-1] Optical Switch
An instance of application of the holographic optical device, as an optical switch, is explained with reference to FIGS. 5A and 5B. This holographic optical device is comprised of a hologram layer, having areas of a high molecular material 425 alternately layered with areas of a positive nematic liquid crystal material 424 (a nematic liquid crystal material in the form of a refractive index ellipsoid having its long axis coincident with the long axis of the liquid crystal molecule), and a pair of glass plates having transparent electrodes 422, 423 and which are arranged for sandwiching the hologram layer in-between, as shown in FIG. 5A, as described in Japanese Laying-Open Patent Publication H-5-173196.
In the absence of an electrical voltage across the transparent electrodes, as shown in FIG. 5A, the nematic liquid crystal material (liquid crystal molecules) 424 is oriented perpendicularly to the high molecular material 425, so that diffractive effects are produced by periodic variations in the refractive index as concerns the incident light of the orientation of polarization which becomes the extraordinary light with respect to the nematic liquid crystal material 424.
On the other hand, as shown in FIG. 5B, if the voltage is applied across the transparent electrodes 422, 423 to render the long axis of the nematic liquid crystal material 424 parallel to the high molecular material 425, the incident light of the orientation of polarization which acts as the extraordinary light with respect to the nematic liquid crystal material 424 in FIG. 5A becomes the ordinary light with respect to the nematic liquid crystal material 424 to produce no difference in the refractive index with respect to the high molecular material 425, so that no phenomenon of diffraction is produced.
By the above principle, the holographic optical device is able to operate as a light switch by controlling the applied voltage.
[6-2] Reflecting Plate for Image Display Device
By way of another instance of application of the holographic optical device, as the reflecting plate for image display device, a light beam 504 incident from outside to a direct viewing reflection type liquid crystal panel 502 is reflected by a holographic reflecting plate 503 in an orientation 506 different from the regular reflecting direction to display an image with optimum contrast, as reflected light 505 from the surface of the direct viewing reflection type liquid crystal panel 502 is prevented from falling on a pupil 507 of a viewer, as shown in FIG. 6 and as disclosed in Japanese Laying-Open Patent Publication H-9-138396. Meanwhile, the hologram in this case may not be a polarization type hologram.
[6-3] Polarization Converter for Projection Type Image Display Apparatus
In an instance of application of the holographic optical device to a polarization converter for a projection type image display device, the illuminating light radiated from a light source 610 is radiated as a substantially collimated light beam in one direction by an reflection plate 612 processed e.g., with vapor aluminum deposition, as shown in FIG. 7 and as disclosed in Japanese Laying-Open Patent Publication H-8-234143. The illuminating light is diffused after transmission through a diffuser 615 to fall on a lenticular array 616, in order to reduce brightness fluctuations of the illuminating light on a liquid crystal display (LCD) 614 and in order to improve the illumination efficiency based e.g., on the rectangle conversion function proper to the lenticular array 616.
The illuminating light then falls on a transmission type polarization selective holographic optical device 618. By the above-described function of the transmission type polarization selective holographic optical device 618, the respective components of the P-polarized light and the S-polarized light are separated from each other depending on the angle of incidence. The illuminating light then falls on a patterned half wave plate array 620. In this half wave plate array 620, the component of orientation of polarization of the P-polarized light or the S-polarized light of the illuminating light, which is perpendicular to the orientation of the incident polarized light on the LCD 614, traverses the patterned half wave plate part of the half wave plate array 620 to change the orientation of polarization by 90xc2x0.
With this holographic optical device, it is intended to improve the utilization efficiency of the illuminating light radiated by the light source 610.
In connection with the above-described image display device and the image display apparatus, the problem to be tackled by the present invention is as follows.
(1) First, if the polarization beam splitter PBS is used for illuminating the reflection type spatial light modulator, as in a projection type image display apparatus employing the reflection type spatial light modulator, shown in FIG. 2, this PBS is of a cubic form having a side longer than the long side of the image display part of the reflection type spatial light modulator, so that the distance between the reflection type spatial light modulator and the projection optical system, that is the back-focus of the projection optical system, cannot be reduced. If the back-focus is prolonged, it becomes difficult to reduce the F-number, that is, to obtain a bright lens. So, with the present image display apparatus, the illuminating light emitted from the light source is low in utilization efficiency.
Moreover, with the present image display apparatus, employing the PBS, it is difficult to reduce the size of the apparatus, while it is difficult to reduce the weight of the apparatus because this PBS is formed of glass. Since this PBS needs to be formed of a high quality glass material of low birefringence and thermal distortion, and a multi-layered dielectric film is used for separating the P-polarized light from the S-polarized light, it is a costly material, such that it is difficult to lower the production cost of the entire image display apparatus. On the other hand, with the image display apparatus, formed using this PBS, it is difficult to achieve image display with high contrast, high uniformity and high color reproducing characteristics.
(2) As means for overcoming the above-described problem, there is proposed an illuminating apparatus for a reflection type spatial light modulator not employing the PBS, as shown in FIG. 3. However, the image display apparatus, shown in FIG. 3, suffers from the following problem: That is, since a holographic optical device 303 provided on the window surface side (incident/outgoing side) of the reflection type spatial light modulator is not a polarization selective holographic optical device, but is a polarization dependent holographic device, the light utilization efficiency is not optimum.
It is because this holographic optical device is not provided with a layer showing birefringence characteristics in its layer forming a periodic structure of the fluctuations in the refractive index, so that it is impossible to reduce the refractive index of the P-polarized light or the S-polarized light to zero.
Moreover, in this image display apparatus, such a technique is proposed in which, in order to suppress the diffraction efficiency of the P-polarized light, used as the illuminating light for image demonstration to as low a value as possible to prohibit the light from being returned again on diffraction towards the illuminating light source, the S-polarized illuminating light, diffracted by the holographic optical device, is caused to fall on the reflection type spatial light modulator from an oblique direction relative to the perpendicular direction to differentiate the angle of re-incidence of the reflected light converted into the p-polarized light on the holographic optical device from the angle of incidence at the time of first incidence to set a state not in meeting with the conditions of diffraction.
However, in this case, since the reflected light from the reflection type spatial light modulator is radiated at an angle relative to the perpendicular direction to collapse the telecentricity, there arises the necessity in a routine co-axial projection optical system for increasing the image cycle of the optical system in order to prevent the efficiency from being lowered. The increased image cycle of the optical system leads to an increased size and cost of the apparatus. Moreover, if, in the routine reflection type spatial light modulator, the incidence angle of the light ray deviates from the perpendicular direction, contrast is deteriorated in many cases. So, in this image display apparatus, high contrast image display cannot be achieved.
Before tackling this problem, there is encountered a problem that, in the present image display apparatus, it is extremely difficult to set a state in which the P-polarized component be not in meeting with the condition of diffraction. That is, in the present image display apparatus, the center of the hologram lens of the hologram color filter is offset by approximately 0.5 times the hologram lens size from the center of the pixel electrode of the reflection type spatial light modulator. If, in this case, an angle of incidence xcex8in of the main light beam of each hologram lens is
xe2x80x83xcex8in=ArcTan [r/LP]
where r is the radius of the hologram lens and Lp is the distance in the direction of thickness between the hologram lens and an aluminum pixel electrode of the reflection type spatial light modulator, Lp=0.7 mm (supposing that the cover glass thickness is 0.7 mm) and r=10 xcexcm (supposing that the size of one pixel with R, G and B combined together is 20 xcexcm), xcex8in is given by
xcex8in=ArcTan [r/Lp]=0.82xc2x0.
This angle is only small as compared to the angle of diffusion of the illuminating light incident on the hologram color filter (on the order of xc2x110xc2x0), such that, if the angular difference between the P-polarized light and the S-polarized light is as small as 1.64xc2x0 (=0.82xc3x972), it is extremely difficult to distinguish the two based on the angle of incidence.
If the range of the allowed diffraction angle of the hologram color filter is 1xc2x0 to 2xc2x0, the polarization separation characteristics are improved, however, the light volume of the illuminating light with the angle of diffusion of xc2x110xc2x0 actually diffracted and which may be effectively used is extremely small. Therefore, this solution cannot be said to be realistic.
On the other hand, if desired to exploit the polarization dependency of the holographic optical device, it is necessary to set the S-polarized light so that it will be the incident light beam, that is diffracted light. For preventing the utilization efficiency of the illuminating light or the contrast of the displayed image from being lowered, additional members or optical devices are needed, thus leading to increased production cost and weight of the overall device.
The reason is as follows: Referring to FIG. 8, in an initial state in which, in the process of progressively increasing the hologram thickness from zero sufficient polarization dependency is achieved, the diffraction efficiency with respect to the S-polarized light is increased, whilst that with respect to the P-polarized light is decreased. Subsequently, the hologram thickness d may be increased to increase the diffraction efficiency with respect to the P-polarized light and to decrease that with respect to the S-polarized light.
However, the wavelength dependency and the incidence angle dependency of the diffraction efficiency of the transmission type hologram are increased with the increased hologram thickness. That is, the tolerance of the deviation from a preset wavelength and from a preset incidence angle of the laser on light exposure of the hologram (the allowance in procuring the diffraction efficiency) is decreased to lower the light utilization efficiency.
FIGS. 9 and 10 show the incidence angle dependency of the diffraction efficiency in case the holographic optical device prepared under the conditions of the incidence angle of the object light of 0xc2x0, incidence angle of the reference light of 60xc2x0, an average refractive index of the hologram of 1.52, the modulation factor of the refractive index of the hologram layer of 0.05, the thickness of the hologram layer of 5 xcexcm and the exposure light wavelength of 532 nm, is read out at a reproducing wavelength of 532 nm. FIGS. 9 and 10 show the results as calculated for the thickness of 6 xcexcm and for the thickness of 18 xcexcm, respectively. The incident polarized light is here assumed to be the S-polarized light. It may be seen from above that the incident polarized light in actuality needs to be S-polarized light.
Meanwhile, if the light proceeds from a medium of low refractive index into a medium of a high refractive index, its surface reflectance exhibits polarized light dependency, as shown in FIG. 11. By such polarized light dependency, the surface reflectance in case the P-polarized light and the S-polarized light are incident on the glass with a refractive index of 1.5 in air is larger at all times for the S-polarized light than that for the P-polarized light. Also, if the incidence angle meets tan xcex8=n(=1.5), that is the Brewster""s angle, herein 56.3xc2x0, the reflectance of the P-polarized light is zero. At this time, the reflectance of the S-polarized light is on the order of 15%.
This means that, if the light ray is caused to fall obliquely on the glass substrate of the holographic optical device, by way of off-axis incidence, the light utilization efficiency is higher in case the incident light is the P-polarized light. With the above-described holographic optical device, in which the incident light needs to be the S-polarized light, a coupling prism 305 is used to prevent the efficiency from being lowered, as shown in FIG. 3. However, if this coupling prism is used, the number of the component parts and the weight as well as the cost of the apparatus are increased. Moreover, if the coupling prism is used, surface reflectance cannot be reduced to zero and hence it is not possible to prevent generation of stray light or the deterioration in the contrast of the displayed image reliably.
Also, if the coupling prism is used, the incidence of the light ray from illuminating means on a hologram layer is not other than the angle of view of the light ray radiated by the illuminating means. In the case of a typical projector optical system, the angle of view of the light ray radiated from the illuminating means is on the order of xc2x110xc2x0. It is not that easy to maintain the diffraction efficiency of the holographic optical device at a high constant value.
If the illuminating light from a lamp light source is converged to illuminate image display devices of a certain area, the angle of incidence uxe2x80x2 becomes small in inverse proportion to the size yxe2x80x2 of the image display device, as shown by Lagrange-Helmholtz invariant of the following equation:
ynu=yxe2x80x2nxe2x80x2uxe2x80x2(Lagrange-Helmholtz invariant)
where y is an image height from the optical axis, n is the refractive index of the medium and u is the angle of tilt of the light ray.
The above equation indicates that the value of the product ynu is invariable on any plane of the optical system. That is, if the product ynu of the left side assumes a finite value, and the image display device is reduced in size, the angle of incidence to the image display device is increased further. This represents a factor all the more unfavorable in realizing the holographic optical device of high efficiency. As may be seen from FIG. 9, the diffraction efficiency is lowered to 25% and to approximately 0% if the angle of incidence deviates by +10xc2x0 and by xe2x88x9210xc2x0 from the angle of incidence which gives the high peak value.
Also, in the above-described image display apparatus, the holographic optical device is used at all times as a color filter. Thus, in this image display apparatus, such a step is needed in which micro-sized lenses of a size approximately equal to the pixel size and these micro-sized lenses are brought into correct registration with respective pixels of the liquid crystal display device, thus increasing the production difficulty and the production cost.
Moreover, the above-described image display apparatus is not able to cope with a configuration of so-called field sequential color technique or with a configuration of using plural reflection type image display devices from one color light to another.
In the image display apparatus employing the above-described hologram color filter, spectral separation and collection need to be performed for the incident light from one color light to another, so that color reproducibility or high definition of the displayed image are in a relationship of trade-off with respect to the utilization efficiency of the illuminating light.
This relationship is hereinafter explained. Referring to FIG. 12, the tolerance value of the outgoing angle xcex94xcex8i of the main light beam from the hologram lens, for which the illuminating light is converged on one color pixel, may be found from the following equation:
xcex94xcex8i=ArcTan [r/Lp]
where Lp is the distance between a hologram color filter 700 and a pixel electrode 702 of the reflection type spatial light modulator 701, and 2r is the size of one color pixel electrode, as shown in FIG. 12. If Lp=0.7 mm and r=xc2x15 xcexcm, xcex94xcex8i=xc2x10.4xc2x0.
It is noted that the angle of incidence xcex8c by the interference fringe of the hologram and the outgoing angle xcex8i on diffraction are inter-related by the following equation:
(sin {xcex8s}xe2x88x92sin {xcex8r})/xcex=(sin {xcex8i}xe2x88x92sin {xcex8c})/xcexc
where xcex8s is an incidence angle of the object light at the time of manufacture of the hologram, xcex8r is a incidence angle reference light at the time of manufacture of the hologram, xcex is the design wavelength of the hologram and xcexc is the reproducing wavelength.
From the foregoing, if xcex8s=0xc2x0, xcex8r=60xc2x0, xcex=550 nm, xcexc=550 nm and xcex8i=xc2x10.4xc2x0, xcex8c is 60xc2x10.8xc2x0 C., thus indicating that the tolerance of the angle of incidence to the hologram color filter of the illuminating light beam is extremely narrow. On the other hand, if xcex8s=0xc2x0, xcex8r=60xc2x0, xcex=550 nm, xcex8c=60xc2x0 and xcex8i=xc2x10.4xc2x0, xcex94xcexc=550xc2x14.5 nm, thus indicating that the tolerance of the incident wavelength to the hologram color filter of the illuminating light is extremely narrow.
From the foregoing, high parallelism and a narrow wavelength range are demanded of the illuminating light incident on the hologram color filter, such that, with the use of a routine lamp light source, the light utilization efficiency is lowered significantly owing to the fact that the light radiating part has a finite size on the order of 1 mm and that the light emitting wavelength range is broad. If conversely the light utilization efficiency is to be increased, there is no alternative but to increase the pixel size or to allow for light leakage to the neighboring color pixel. However, the definition of the displayed image and the color purity and reproducibility are lowered in the former and latter measures, respectively.
In the above-described image display apparatus, the holographic optical device cannot be used as a reflection type. If the holographic optical device is used as the reflection type such that the difference between the diffraction efficiency of the P-polarized light and that of the S-polarized light is to be e.g., 30% or higher, d/{circumflex over ( )}, where d is the hologram thickness and {circumflex over ( )} is the pitch of the interference fringes, needs to be on the order of 1.0 to 3.0. For reference, see M. G. Moharam and T. K. Gayload: Rigorous Coupled-Wave Analysis of Planar Grating Diffraction, J. ODt. Soc. Am. 71, 811-818, 1977.
Since {circumflex over ( )}=xcex/|2 sin [(xcex8sxe2x88x92xcex8r)/2]|
where xcex8s is an angle of incidence of the object light and xcex8r is an angle of incidence of the reference light, the minimum value of (xcex8sxe2x88x92xcex8r) is 90xc2x0. At this time, |2 sin [(xcex8sxe2x88x92xcex8r)/2]| assumes a minimum value of 1.41. If xcex=0.5 xcexcm, {circumflex over ( )} assumes a maximum value of 0.35 xcexcm, such that the thickness of the hologram d satisfying d/{circumflex over ( )}=1.0 to 3.0 is 1 xcexcm at the maximum. It is extremely difficult to prepare so thin a hologram layer.
In the above-described various application techniques of the holographic optical devices, there lacks such an application technique in which the illuminating light is incident from an oblique direction to illuminate the reflection type spatial light modulator at a high efficiency.
As a virtual image display optical system, employing the reflection type spatial light modulator, there is such a configuration in which a reflection type spatial light modulator 836, an illuminating light source 834 and a reflecting mirror 842 are arranged in the vicinity of a forming surface of the polarizing beam splitter 848, as shown herein in FIG. 13 and as disclosed in U.S. Pat. No. 5,596,451.
However, in this optical system, a portion 860 of the illuminating light directly reaches an area of observation 846 of the viewer, by the polarizing beam splitter 848, without reaching the reflection type spatial light modulator 836, as may be seen from FIG. 13. This illuminating light may be incident as noise on a pupil 824 of the viewer to raise a fundamental problem of lowering the contrast of the image information displayed by the reflection type spatial light modulator 836.
Moreover, with the present optical system, the entire optical system is of a cubic shape, thus increasing its thickness. If the performance of a dielectric film 864 of the polarizing beam splitter is raised, the production cost is raised. If conversely the dielectric film is lowered in performance, the image is deteriorated in uniformity, especially with pupil movement, due to reflectance of the polarized light by the dielectric film, incidence angle dependency or wavelength dependency of the dielectric film.
For improving this, there is disclosed in Japanese Laying-Open Patent Publication H-11-125791 an image displaying apparatus in which a virtual image displaying optical system is formed using a reflection type spatial light modulator 908 and a free-form surface prism 910, as shown in FIG. 14.
In the image display apparatus, shown herein in FIG. 14, the illuminating light from a light source 912 is directly incident on a reflection type spatial light modulator 908, the reflected light is incident on a third surface 905 of the free-form surface prism 910 to reach the pupil 901 after reflection on the first surface 903 and on the second surface 904 and transmission through the first surface 903 to display a virtual image. This optical system has a drawback that the angle of incidence of the illuminating light on the reflection type spatial light modulator 908 is increased to lower the modulation index of the reflection type spatial light modulator 908 itself to deteriorate the contrast of the displayed image.
On the other hand, an optical system, shown in FIG. 15, in which the illuminating light radiated from the light source 912 through the free-form surface prism 910 is incident on the reflection type spatial light modulator 908 and this reflected light is incident on the free-form surface prism 910 from the thirds surface 905 to reach the pupil 901 after reflection on the first surface 903 and on the second surface 904 and transmission through the first surface 903 to form the virtual image, mainly suffers from the following two problems.
The first problem is as that, if the reflection type spatial light modulator 908 is of the polarization modulation type (phase modulation type), the illuminating light incident on the reflection type spatial light modulator 908 needs to be linear polarized light having a specified orientation of polarization. However, since the free-form surface prism 910 is prepared on injection molding of plastics material, it exhibits birefringence characteristics in its inside. So, the problem is raised that, even if the linear P-polarized light is incident on the free-form surface prism 910, the state of polarization is not maintained, thus deteriorating the contrast of the displayed image. Although this can apparently be evaded by arranging a polarizing plate between the reflection type spatial light modulator 908 and the third surface (refractive surface) 905 of the free-form surface prism 910, the display mode becomes the xe2x80x98normally whitexe2x80x99 mode, thus again deteriorating the contract of the displayed image.
The second problem is produced by the illuminating light directly falling on the free-form surface prism 910 operating as an eyepiece optical system. The illuminating light undergoes inner reflection on the optical surfaces 903 to 905 in the inside of the free curved surface prism 910 to produce stray light. A portion of this stray light reaches the pupil 901 of the viewer thus again deteriorating the contract of the displayed image.
That is, with the illuminating optical system, employing a half mirror, as one of the various image display apparatus, so far proposed, the apparatus cannot be reduced in size, whilst the illuminating light is low in exploitation efficiency. With the illuminating optical system employing a polarizing beam splitter, the apparatus cannot be reduced in size, whilst the displayed image is low in uniformity and the production cost is elevated. With the illuminating optical system for directly illuminating a spatial light modulator and with the illuminating optical system for illuminating a spatial light modulator through an optical component formed of plastics, the displayed image is low in contrast.
In view of the above-described status of the prior art, it is an object of the present invention to provide an image display device and an image display apparatus in which the light exploitation efficiency of the illuminating light is high, the apparatus can be reduced in size and cost and in which the displayed image is uniform and high in contrast.
For resolving the above problem, the present invention provides an image display apparatus including a polarization selective holographic optical device for diffracting illuminating light, the device including a plurality of each of two areas having refractive index values exhibiting respectively different incidence polarization orientation dependencies, the areas being layered sequentially alternately, and a reflection type spatial optical modulator for modulating the state of polarization of the illuminating light diffracted by the polarization selective holographic optical device.
In this image display apparatus, the polarization selective holographic optical device, one of two areas of which exhibits refractive index anisotropy, with the other exhibiting refractive index isotropy, is irradiated with the illuminating light at an angle of incidence not less than 30xc2x0 and less than 90xc2x0 with respect to a normal line to a light receiving surface thereof for the illuminating light. The polarization selective holographic optical device diffracts a P-polarized light component or an S-polarized light component of the illuminating light to radiate the diffracted light towards the reflection type spatial optical modulator. The polarization selective holographic optical device exhibits the diffraction efficiency of not higher than 10% for the polarized light component of the illuminating light incident a second time thereon after phase modulation by the reflection type spatial optical modulator, which polarized light component has a direction of polarization perpendicular to a direction of polarization of the polarized light component diffracted thereby at the time when the illuminating light is first incident thereon, whereby not less than 70% of the first-stated polarization component is transmitted through the optical device.
The image display apparatus of the present invention includes an image display device of the above type, according to the present invention, a light source for radiating the illuminating light, an illuminating optical system for causing the illuminating light radiated by the light source to be incident on a polarization selective holographic optical device of the image display device, and a projection optical system for projecting the illuminating light through the reflection type spatial optical modulator and the polarization selective holographic optical device of the image display device on a screen.
With the present image display device, the polarization selective holographic optical device, one of two areas of which exhibits refractive index anisotropy, with the other exhibiting refractive index isotropy, is irradiated by the illuminating optical system with the illuminating light at an angle of incidence not less than 30xc2x0 and less than 90xc2x0 with respect to a normal line to a normal line to a light receiving surface thereof for the illuminating light. The polarization selective holographic optical device diffracts a P-polarized light component or an S-polarized light component of the illuminating light to radiate the diffracted light towards the reflection type spatial optical modulator. The polarization selective holographic optical device exhibits the diffraction efficiency of not higher than 10% for the polarized light component of the illuminating light incident a second time thereon after phase modulation by the reflection type spatial optical modulator, which polarized light component has a direction of polarization perpendicular to a direction of polarization of the polarized light component diffracted thereby at the time when the illuminating light is first incident thereon, whereby not less than 70% of the first-stated polarization component is transmitted through the optical device. The projection optical system projects the light transmitted through the polarization selective holographic optical device on the screen.
In the image display apparatus of the present invention, there are provided a polarization selective holographic optical device for diffracting the incident light, the device including a plurality of each of two areas having refractive index values exhibiting respectively different incidence polarization orientation dependencies, the areas being layered sequentially alternately, color separation means for separating the illuminating light in a plurality of representing different wavelength range components, an illuminating optical system for causing the illuminating light separated into respective different wavelength range components to be incident on the polarization selective holographic optical device, a plurality of reflection type spatial optical modulators for modulating polarized states of a plurality of representing different wavelength range components of the illuminating light diffracted by the polarization selective holographic optical device, color synthesis means for synthesizing illuminating light portions of respective different wavelength ranges modulated by the plural reflection type spatial optical modulators, and a projection optical system for projecting the illuminating light through the color synthesis means. With the present image display apparatus, the projection optical system projects the illuminating light transmitted through the polarization selective holographic optical device and through color synthesis means on the screen.
The image display apparatus according to the present invention includes wavelength-band-based polarization separating means for separating the states of polarization of respective different first and second wavelength range components of the illuminating light as linear polarized components perpendicular to each other, an illuminating optical system for causing the illuminating light separated into first and second wavelength range components to be incident on the polarization selective holographic optical device, a first reflection type spatial optical modulator for modulating the state of polarization of the first wavelength range component of the illuminating light diffracted by the polarization selective holographic optical device, a second reflection type spatial optical modulator for modulating the state of polarization of the second wavelength range component of the illuminating light transmitted through the polarization selective holographic optical device, and a projection optical system for projecting the illuminating light through the reflection type spatial optical modulator on a screen. The projection optical system projects the illuminating light of the first wavelength range component through the first reflection type spatial optical modulator and the polarization selective holographic optical device and the illuminating light of the second wavelength range component through the second reflection type spatial optical modulator and the polarization selective holographic optical device on the screen.
In the image display apparatus according to the present invention, an optical system for observing a virtual image, designed for guiding the illuminating light through the reflection type spatial optical modulator to the pupil of the viewer, is provided in place of the projection optical system. This optical system for observing a virtual image guides the light transmitted through the polarization selective holographic optical device to the pupil of the viewer.